The recording of radiation images is carried out in various fields. For example, X-ray images have long been used in making medical diagnoses involving investigation of a diseased body part or an injury. Portal imaging devices have been used for the verification of beam placement and patient positioning during radiotherapy. Also, radiation images have been used in industrial applications such as for non-destructive inspection of substances.
Prior techniques for recording x-ray images have included storing the image on a photostimulable phosphor sheet and then exposing the sheet to stimulating energy, such as from a laser beam. The photostimulable phosphor sheet consists of a powder of small photostimulable phosphor crystals bound to a plastic sheet with a clear binder. The laser radiation causes the sheet to emit light in proportion to the amount of energy stored when the sheet is exposed to the x-ray radiation through a process known as photostimulable luminescence (PSL). The light emitted by the photostimulable phosphor sheet is photoelectrically detected and converted into an electric image signal. The electric image signal is used to reproduce and store a digital image which can then be displayed as a visible image, such as on a cathode ray tube (CRT) display device or a liquid crystal (LCD).
Other techniques include capturing the x-ray image using a scintillating screen consisting of a powdered phosphor such as gadolinium oxysulfide (Gd2O2S) or a structured phosphor such as cesium iodide activated by thallium (CsI:Tl), both of which promptly convert the x-ray image into a visible light image. This image of visible light is then typically directed onto a detector array such as a charge-coupled device (CCD) from which a digital image can be read for immediate display or storage. Various techniques utilizing mirrors, lenses, and fiber optic components have been developed to demagnify the light image from the field of view in a patient (typically 35×43 cm) to the size of commercially available CCDs (approximately 5×5 cm or smaller). In some cases, clusters of CCDs (e.g. 2×2) have been used to reduce the demagnification required. These configurations are collectively referred to herein as the screen/CCD technique.
Still other prior techniques involved capturing the x-ray image in a clear thick scintillating crystal plate by coupling the scintillator plate, such as CsI:Tl, to a slow-scan cooled CCD camera with a high-speed lens (referred to herein as the plate/lens/CCD technique).
Although systems based on some of these techniques are commercially available, performance limits leave ample room for improvements, and trade-offs between critical image quality parameters continue to be required.
A fundamental limitation of PSL-based systems is the scattering or diffusion of the scanning laser beam as it penetrates deeper into the photostimulable phosphor sheet causing a degradation in the spatial resolution of the final image. Consequently, a trade-off must be made between quantum detection efficiency (QDE) which can be increased by making the phosphor sheet thicker and the modulation transfer function (MTF) which is improved by making the phosphor sheet thinner.
Systems incorporating the screen/CCD or the plate/lens/CCD techniques encounter a fundamental limitation due to the image demagnification required which results in low light collection efficiency causing a secondary quantum sink at this point in the imaging chain. Even using the largest CCD chips and fastest lenses available, it is not possible to collect more than about 0.2% of the light emitted by the scintillator plate under x-ray bombardment. The consequence of this inherent limitation is reduced image quality, such that for diagnostic x-ray imaging, the performance of these devices is inferior to both traditional screen-film and PSL-based systems.
Even in portal imaging applications, the plate/lens/CCD technique has some severe disadvantages. (1) The CCD device required is large, expensive, and sensitive to radiation, thus requiring heavy and bulky radiation shielding. (2) The fast lens needed for x-ray quantum limited performance has a limited depth of focus which causes a trade off between QDE and MTF. Although this tradeoff is far less severe than it is for conventional and photostimulable powder phosphor screens, it is still present and puts a limit on the thickness of the crystal plate that can be used and still achieve high spatial resolution. (3) The CCD camera has a limited well depth, and many dozens of images would have to be acquired during a radiation exposure to prevent saturation. To prevent loss of any of the available dose for imaging, two CCD cameras or a single frame transfer CCD camera would be needed, adding to the expense and size of the system. (4) The CCD camera integrates the light emission data which has to be read out during the radiation exposure. This makes the CCD camera output subject to the effects of any radiation reaching the camera from the x-ray accelerator. Since even the best shielding can reduce the radiation exposure to the camera by only about a factor of 4, radiation noise would be a major problem for a plate/lens/CCD system. (5) To achieve the best spatial resolution, the CCD camera has to view the scintillator plate from the entrance side. This requires a 45 degree mirror, which means that the patient has to be quite far away from the scintillator plate. This reduces the spatial resolution due to the accelerator focal spot size and increases the required size of the expensive scintillator plate needed to achieve an adequate field of view.
What is needed, therefore, is an x-ray imaging system having better spatial resolution, higher quantum efficiency and higher signal-to-noise ratio, and which contributes little or no radiation noise.